The invention relates to a monopolar electrolytic cell suitable for electrochemical processes carried out with periodic reversal of the polarity. The periodic polarity reversal of electrochemical cells, whereby each of the electrodes works alternatively as anode and as cathode for preset intervals of time, is a measure known in the art especially for preventing the formation of scaling of various kind on the surface of one of the electrodes, usually the cathode. The above for example is the typical case of cells used for electrolysing diluted alkaline brines to produce active chlorine (that is, a mixture of hypochlorite and hypochlorous acid with possible traces of dissolved free chlorine and other species at equilibrium) at the anode: especially in case brine is obtained from tap water, containing carbonates and other anions of similar behaviour, the cathode becomes a site of preferential deposition of carbonates and other insoluble salts, which is favoured by process-induced alkalinisation nearby. Such deposits negatively affect current transmission by the electrode, whose electrical efficiency may degrade irreversibly in time. The periodic reversal of current direction and thus of electrode polarity makes the surface working cathodically for a half cycle to start functioning as the anode upon reversal, being subject to a local acidification which favours dissolution of the precipitate previously formed. Other electrolytic processes sometimes subject to periodic current reversal are for instance the treatment of waste waters containing organic substances, which are degraded at the anode while various kinds of deposits tend to be formed at the cathode, or cathodic deposition of metals from electrolytic baths with simultaneous anodic degradation of organics, used for treating waters in which both types of species are present as impurities. In such cases, also the anode is often subjected to the deposition of polluting films, in this case consisting of organic residues which tend to oligomerise upon the electrode surface, and which sometimes may be removed by the mechanical and chemical action of nascent hydrogen in the subsequent cathodic cycle. For the sake of preserving the regularity of operation and maintaining operative parameters of the desired process constant, the electrodes installed in the cells, destined to work alternatingly as anodes and as cathodes, besides being spaced at constant gap must preferably be of the same size, so that it is possible to keep both current supplied and operating voltage constant (except for the change of sign). This implies that the cell design for this type of processes is mainly limited to planar-type geometries, in other words contemplating the use of pairs of facing planar electrodes. However, in many cases this can constitute an undesired limitation, involving some negative consequences. In many cases in fact this kind of processes is carried out in small size units, such as the case of active chlorine production for disinfection of waters to be used in hospital, hotel or domestic field, or in the recovery of precious metals in jewellery wastes. For such kind of applications it can be important to limit volumes inasmuch as possible, selecting cell designs of coaxial concentric type, for instance cylindrical cells with outer cathode wall and central anode. This can have the advantage, besides a better exploitation of the available volume, of improving current transmission minimising edge effects, which are known to be heavier in planar geometries and very relevant in case of overall electrode areas of small size. Cells of coaxial concentric type, both cylindrical or prismatic, are characterised however by having an external electrode of bigger size than the internal one, making operation with periodic current reversal more difficult. Keeping constant in fact current intensity between one cycle and the next and thus the production of the desired species, the variation of the corresponding electrode area would entail a corresponding variation of current density and hence of process voltage; on the other hand, should one decide to operate at constant voltage, current intensity and hence production rate would oscillate between two values corresponding to the two different electrode areas, hardly in agreement with the normal requirements of an industrial process.
It was therefore identified the need for providing electrolytic cells of concentric electrode geometry, with constant interelectrode gap and with cathode area identical to the anode area.